An invasive nematode has harmed eel populations in the U.S. Southeast, so that may be a cause, Pientka said. He said other fish species in the lake do not seem to be affected, so low oxygen levels or pollution are likely not factors.

Adult eels have a complicated life cycle. They grow up to three feet long, and then migrate to the Sargasso Sea in the Atlantic Ocean in the late summer and fall to spawn. The migrating eels undergo physiological changes to adapt to salt water and the ocean depths.

“They are thought to use some of the deep water currents to get to the spawning areas, so they have to basically adjust to that higher pressure,” Pientka said.